This application claims the priority of German Patent Document 198 44 213.0 filed in Germany on Sep. 26, 1998 and PCT/EP99/06343 filed in Europe on Aug. 28, 1999.
The invention relates to a method for the closed-loop or open-loop control of a forced-induction internal combustion engine.
German Reference DE 40 25 901 C1 has disclosed an exhaust turbocharger for an internal combustion engine that has a turbine with a turbine geometry that can be varied by means of variable turbine guide vanes and a compressor driven by the turbine for increasing the boost pressure in the cylinder inlet. The turbine guide vanes can be adjusted by an actuator so that the effective turbine cross section of the turbine is modified. This makes it possible to achieve different exhaust backpressures in the section between the cylinders and the exhaust turbocharger, depending on the operating state of the internal combustion engine, thereby allowing the output of the turbine and the output of the compressor to be adjusted according to requirements. The turbine guide vanes are adjusted to a desired boost pressure in accordance with specified characteristics.
In order to achieve an improvement in efficiency in a simple manner during nonsteady-state operation of the internal combustion engine, boost-pressure control is performed in accordance with different characteristics above and below a threshold value for the exhaust backpressure. This makes it possible to prevent the occurrence of uncontrolled increases in pressure in the exhaust line upstream of the turbine while the boost pressure is still rising after a positive load change. The internal combustion engine no longer has to expel the exhaust against an increased exhaust backpressure and efficiency is increased.
Another method for closed-loop control of the boost pressure is known from German Reference DE 195 31 871 C1. In order to improve efficiency during nonsteady-state operation of the internal combustion engine, especially after a positive load change from low load and engine-speed ranges, this publication proposes to determine the difference between the exhaust backpressure and the boost pressure as the controlled variable for closed-loop control in order to adjust the boost pressure. This makes it possible to detect an impermissibly high deviation in the exhaust backpressure in the case of a positive load change and to correct it by suitable measures. measures.
The problem underlying the invention is to optimize the operating behaviour of the engine over all load and engine-speed ranges.
According to the invention, open-loop control is exercised in the lower load/engine-speed range and closed-loop control is exercised in the upper load/engine-speed range. The division into an upper and a lower load/engine-speed range with the respectively assigned characteristic maps has the advantage that it is possible to decide, by comparison with a defined specifiable limiting value, whether open-loop or closed-loop control should be performed. By distinguishing between open-loop and closed-loop control, it is possible to follow an optimum strategy appropriate to the respective operating situation while allowing for partially contradictory aims. By switching between closed-loop and open-loop control according to requirements, the operating behaviour of the internal combustion engine can be optimized with regard to fuel consumption and dynamic response.
In the lower load/engine-speed range, rapid adjustment of the variable turbine geometry without delay is possible owing to the directness of control, thereby improving response. A further advantage is that open-loop control is not dependent on the pressure conditions in the exhaust line or in the intake section, making it possible to eliminate external influences, e.g. falling atmospheric pressure in operation at altitude, and to set the same position of the turbine geometry under all conditions. In the lower load/engine-speed range, a characteristic map incorporating the positions for the turbine geometry is expediently specified. Closed-loop control, which would be problematic in any case owing to the low energy potential because, in the low-pressure range, the boost pressure is almost independent of the exhaust backpressure and consequently independent of the position of the variable turbine geometry, is not required.
Open-loop control also has advantages in terms of fuel consumption compared with closed-loop adjustment in the lower load/engine-speed range. Open-loop specification of the turbine""s position avoids a situation where the turbine geometry is moved into the pressure build-up position and the exhaust backpressure upstream of the turbine is increased. This avoids the need to increase fuel injection to allow the exhaust to be expelled into the exhaust line against the exhaust backpressure.
In the upper load/engine-speed range, on the other hand, the pressure level is higher, favouring closed-loop control and thus also allowing changing external influences to be taken into account. At this pressure level, the boost pressure rises steeply as a function of the actuating travel of the turbine geometry, so that even small changes in the actuating travel lead to a large change in the boost pressure. In this case, open-loop control would be too inaccurate because dimensional inaccuracies due to wear, thermal expansion and forced-induction would lead to an impermissibly large deviation from the desired value. Closed-loop control can compensate for such inaccuracies.
In addition to the possibility of optimizing the characteristic maps for the different ranges with regard to the criteria of fuel consumption and dynamic response, further distinguishing features, such as a steady-state/nonsteady-state engine operating state, can furthermore be taken into account and additional characteristic maps can be assigned to these operating states, thus not only allowing a distinction to be drawn between the upper and the lower load/engine-speed range with the associated characteristic maps but also further characteristic maps to be taken into account in accordance with the additional criteria.
In a preferred embodiment, the position of the variable turbine geometry is stored as a function of load and engine speed in the associated characteristic map in the lower load/engine-speed range. For this purpose, either the position of the turbine geometry or actuating-signal values that are fed to the actuator acting upon the turbine geometry can be specified.
For closed-loop control in the upper load/engine-speed range, desired boost-pressure values are preferably specified as a function of the load and engine speed. For closed-loop control, the position of the variable turbine geometry is varied until the actual boost-pressure values coincide with the desired values.
As an alternative, closed-loop control can also be performed by way of the engine speed. In this case, desired engine-speed values are stored in the characteristic map as a function of the load and compared with actual engine-speed values. Setting of the desired values is performed by varying the position of the turbine geometry in a similar manner to that for boost-pressure control.